1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to an improvement of a bead portion structure in such a pneumatic tire which facilitates the increase and tuning of rigidity required in the bead portion and hence the tire itself and simultaneously establishes the steering stability and the ride comfort in a high level.
2. Description of Related Art
Heretofore, it has widely been conducted to embed an additional reinforcing member such as wire insert, flipper or the like in the bead portion for the increase and tuning of the rigidity in the bead portion of the pneumatic tire.
However, when the rigidity of the bead portion is increased by such an additional reinforcing member, for example, a wire insert wherein the wire extends obliquely in a constant direction crossing with a circumferential line segment viewing at a side face of the tire, the longitudinal rigidity of the bead portion in the tire is increased in addition to the rigidity in front and back directions, so that the steering stability is improved but the ride comfort is largely degraded.
It is, therefore, an object of the invention to solve the aforementioned problems of the conventional technique and to provide a pneumatic tire capable of simultaneously establishing the steering stability and ride comfort in a higher level by adequately selecting a winding form of a bead wire, which develops to fix an end portion of a carcass ply and adhere the bead portion of the tire to a rim sheet and the like, to simply and easily increase or decrease the rigidity required in the bead portion as being expected without using the additional reinforcing member such as wire insert, flipper or the like.
According to the invention, there is the provision of a pneumatic tire having a bead portion structure that at least two bead wire structural bodies, each being obtained by spirally winding a bead wire(s) so as to continuously extend in a circumferential direction, are disposed in a bead portion so as to sandwich a carcass ply or enclose with the carcass ply, in which an outer end of at least one of the bead wire structural bodies in a radial direction is located on a position corresponding to a rim line or outward therefrom.
According to such a bead portion structure in the tire, the bead wire is spirally wound up to the position corresponding to the rim line or outward therefrom in the radial direction of the tire, whereby the rigidity of the bead portion and hence the tire in front and back directions can be increased to advantageously improve the steering stability.
Furthermore, the spirally wound bead wire continuously extends in the circumferential direction and has no component extending in a direction crossing with a circumferential line segment as in the aforementioned wire insert, so that the increase of longitudinal rigidity can effectively be prevented with the increase of the rigidity in the front and back directions of the tire to ensure an excellent ride comfort.
In a preferable embodiment of the invention, the outer end of at least one of the bead wire structural bodies is positioned between the position corresponding to the rim line and a position of a maximum tire width.
According to this embodiment, the rigidity required in the tire or rigidity in the front and back directions is sufficiently increased and also the increase of the longitudinal rigidity can advantageously be controlled. In other words, if the outer end of the bead wire structural body in the radial direction is located over the maximum tire width position, the rigidity of a sidewall portion is increased and hence the longitudinal rigidity of the tire is too increased to degrade the ride comfort.
In another preferable embodiment of the invention, the rigidity in a lateral direction of the tire is changed between the mutual bead wire structural bodies located at inside and outside in the widthwise direction of the tire. Thus, the rigidity of the tire in the front and back directions can easily and properly be tuned in accordance with the required performances without excessively reinforcing the bead portion, whereby the reductions of tire weight and cost can be realized.
For instance, when the rigidity to lateral input directing inward in the widthwise direction is made high and the rigidity to lateral input directing outward in the widthwise direction is made low to make stress created in the deformation of the bead portion outward in the widthwise direction of the tire small and inversely make stress created in the deformation inward in the widthwise direction large, longitudinal spring constant in the riding over projection or the like so as to deform both bead portions outward in the widthwise direction can be controlled to a small value to ensure the excellent ride comfort.
On the other hand, in the cornering or the like so as to deform one of the bead portion inward and the other bead portion outward in the widthwise direction, the excellent steering stability can be ensured by the high rigidity of the bead portion to the inward deformation.
On the contrary, when the rigidity to lateral input directing inward in the widthwise direction is made low and the rigidity to lateral input directing outward in the widthwise direction is made high, the lateral rigidity of the tire can advantageously be controlled to more improve the steering stability. In this case, the longitudinal rigidity of the tire is also increased in the riding over the projection or the like to deny the degradation of the ride comfort, so that the adoption of this structure is useful when the improvement of the steering stability is particularly important.
In order to simply provide the required rigidity difference, it is favorable that the change in the rigidity of the bead wire structural body in the lateral direction of the tire is caused by at least one means applied to each bead portion between the mutual bead wire structural bodies located at inside and outside in the widthwise direction of the tire, which is selected from means for changing a diameter of the bead wire, means for changing an end count of bead wire in the bead wire structural body, means for changing the outer end position of the bead wire structural body in the radial direction and means for changing the construction of the bead wire, for example, using a combination of single wire and cable of twisted wires.
In the other preferable embodiment of the invention, the rigidity of each bead wire structural body in the longitudinal direction of the tire is changed in the radial direction. More particularly, the rigidity is made smaller in the outer portion of the bead wire structural body in the radial direction.
According to this embodiment, the rigidity of each of two or more bead wire structural bodies is changed together in the radial direction of the tire, whereby the balance between the rigidity in the front and back directions and the longitudinal rigidity in the tire can be controlled in a higher level and hence the high steering stability can be ensured while realizing the excellent ride comfort under the more effective control of the longitudinal rigidity.
And also, the sudden change of the rigidity in each direction ranging from the bead portion to the sidewall portion can advantageously be mitigated.
It is favorable that such a rigidity change is caused by at least one means selected from means for changing a bead wire diameter in the bead wire structural body in the radial direction, means for changing an arranging pitch of the bead wire and means for changing the construction of the bead wire.